Data processing machines do not talk to each other by telephone the same way that people do. Instead of voices, machines use code to communicate information. 5280 communicating devices use standard computer codes, such as ASCII and EBCDIC.
For machines to talk to each other, they must speak the same language. This means that the machines must use the same code, have the same type of communications features, and operate at the same speed. Speed is measured in bits per second (bps).
In addition to code, feature type, and speed, data communications are also dependent on distance. People talking close to each other usually do not need a microphone, an intercom, or a telephone. As the distance between people gets greater, however, such instruments become necessary.
Communications features are designed for installations where the machines that must exchange data are far enough apart (remote) to
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Modems
Distance between a 5280 communicating device and another system also affects the quality of the electrical signals that carry the code.
The longer the line, the weaker the signals due to the resistance of the line. This is one reason why data communications networks usually make use of modems. A modem is a machine that connects a communicating device to a communications line.
The signals produced by a communicating device are weak and generally not in the form used by communications lines. A modem amplifies the signals and changes them into a form that the
communications line can handle. At the receiving end of the
communications line, another modem changes the signals back into the form used by the data processing machine.
Although modems are needed for most 5280 communications features, it is possibie to connect two devices without a modem by using the Digital Data Service Adapter (DDSA) feature. This type of connection is described in this chapter under 5280 Data Communications
Equipment.
Communications Lines
A communications line is used to transfer information from one location to another. A line that is used to transfer information in one direction for a while, and then in the opposite direction for a while, is called a half-duplex line. Changing the direction of transmission is referred to as turnaround, and this requires a certain amount of time.
To reduce turnaround time, a full-duplex line is sometimes used. A full-duplex line allows information to be transferred in both directions at the same time.
5280 communications features are designed to operate in a half-duplex mode, but can be connected to a full-duplex line. When connected to a full-duplex line, 5280 communicating devices do not transfer
information in both directions at the same time, but turnaround time is reduced.
Communications lines can be either switched or nonswitched.
Switching is done by dialing, and the lines are generally owned by a communications company. (A communications company is also called a common carrier; a telephone company often sen/es in this ro!e.) Nonswitched lines can be privately owned or leased from a communications company.
Networks
Communications lines, modems, and other communications equipment can be arranged in several ways, depending on the intended use. The different arrangements of this equipment are called networks. The basic types of networks are:
• Point-to-point, nonswitched
• Point-to-point, switched
• Multipoint (nonswitched)
Following are illustrations of these basic types of networks:
Point-to-Point Network, Nonswitched
• 5280 communicating device connected to another system
• Nonswitched, permanent connection
Other system 1---15280 communicating device
----Point-to-Point Network, Switched
• Any or all remote devices can be a 5280 communicating device
• Switched, temporary connection to one device at a time
• Connection only occurs during actual communication
I
ModemH Remote devicelOther systemt----IModem L-""""'4IIIlr---,~;;;;,;;~---IRemote device
I
Modem HRemote device'72
Multipoint Network (Nons witched)
• Any or all remote devices can be a 5280 communicating device
• Nonswitched, permanent connection
• Other system calls (polls) remote devices by their unique address
• Only the called device answers
• Other remote devices wait their turn for a call
Ir--_ ....
,Modem t-I - -...1
Remote device r IOther systemH Modem f---z--o'r---lMooonJt---.... (Remote deviceI
1....-_"", Modem t - - -... Remote device
For a more complete introduction to the basic principles and components of data communications, see the general information manual, Data Communications Concepts.
,
WHAT TO DO WITH THIS CHAPTERNow with a basic understanding of data communications, you are ready to arrange for the necessary communications equipment. The main steps you should complete are:
• Meet with your IBM representative or someone in your company who is responsible for communications networks.
• Get an exact list of the 5280 communications features that have been ordered.
• Determine the type of network to be established (refer to the following pages on system design for communications).
• Identify the type of communications connection and programs to be used.
• Meet with someone from a communications company and place an order for equipment, services, or both.
• Fill in your responses to the questions listed at the end of this chapter.
System Design for Data Communications
The first step in. defining a communications network involves
documenting the application parameters. This information should be gathered through consultations with the system analyst, system programmer, and/or any host system personnel:
• Number and locations of communicating machines
• Information flow patterns between programmable data stations, programmable control units, and/or main processors
• Types of transactions
• Volume of each type of transaction
• Urgency of the information to be transmitted
• Number of stations that will simultaneously require access
• Reservations for increases in transaction volumes, types
• Reliability and availability requirements
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Host Considerations
Host system planning must begin early in the planning cycle. By notifying the affected personnel early, time can be allotted for programming or other changes required to support the 5280. The following items should be defined:
• Access method (for example, BTAM, VTAM, or TCAM)
• Security procedures
• Error-detection procedures
• File-sharing arrangements
• Program storage requirements
• Network addresses and identification procedures
Compatibility
Experience has proven that compatibility among components is a major consideration for new installations. To reduce delays caused by incompatibility, we recommend that you consult your IBM
representative and, if necessary, a representative of a communications common carrier company. These representatives can aid you in identifying any trade-offs among alternatives, and can review your selection of facilities. The following factors must be identical at both ends of a communications connection.
Transmission protocol: Switched or nonswitched Full- or half-duplex
600, 1200, 2400, or 4800 bits per second Integrated or external auto or manual answer internal (machine) or external (modem) clocking
Batch, inquiry, or RJE
Your planning schedule should reflect the dates of any meetings with IBM representatives or communications company personnel.
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THE 5280 COMMUNICATIONS PROGRAM PRODUCT
The Communications Program Product consists of communications access method (CAM) support, a set of communications utility
programs, and the support for user-written communications programs.
The Communications Reference Manual, provides information about 5280 data communications.
The CAM provides a necessary link between a communications program, either IBM-supplied or your own, and the communications network. With the CAM, the utility programs can perform the following data transfer functions in either a BSC or SNA/SDLC network:
• Send data to a terminal or host computer
• Receive data from a terminal or host computer
• Inquire into host subsystem files and receive replies
• Submit, execute, and obtain results of jobs from a host subsystem The utility programs can be divided into two groups: those that are used to prepare for communications (called preparation utilities), and those that actually transfer data over a communications line (called execution utilities). Storage requirements for the utility programs appear in Appendix E.
Preparation Utilities
The preparation utilities consist of interactive programs in which you specify the characteristics of your execution utilities and
communications configuration. Your responses to the prompts are saved on diskette for convenient reuse. The preparation utilities and their purposes are as follows:
• The communications configuration utility is used to describe the communications environment.
• The data communications description utility is used to describe transmit, receive, and inquiry functions for the following execution utilities: BSC batch transfer, BSC data communications, and SNA data communications.
• The RJ E description utility is used to describe jobs for the MULTI-LEAVING remote job entry and SNA remote job entry utilities.
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Execution Utilities
Each execution utility is specifically designed for and can be used in only one type of network, either BSC or SNA/SOLC. The utility programs can also communicate with certain IBM systems and host subsystems.
• The BSC batch transfer utility enables the 5280 to send and receive batch data to and from diskette over communications lines.
• The BSC data communications utility enables the 5280 to send and receive batch data, and to send inquiries and receive replies from a host subsystem.
• The MULTI-LEAVING remote job entry utility enables the 5280 to submit, execute, and obtain results of jobs from a host RJE
subsystem using BSe link protocol.
• The SNA data communications utility transmits and receives batch data and sends inquiries and receives replies from a host
subsystem. Up to four LU-LU sessions can operate concurrently using the same CAM.
• The SNA remote job entry utility enables the 5280 to submit, execute, and obtain results of jobs from a host RJE subsystem using SOLC link protocol.
r
The accompanying chart shows which utility programs of the program product can be used to communicate with IBM systems and host subsystems.
With Binary Synchronous Communications, the 5280:
Can appear to be a: When it uses To communicate with 3741 Work Station BSC Data 5/3 with CCP or RPG II
Communications Utility 5/32 with RPG II
or 5/34 with RPG II or SSP-ICF
Communications Terminal Communications Utility DOS/VSE POWER System/3 MRJE MULTI-LEAVING System/370 (RJE):
Work Station Remote Job Entry OS/VS1 RES
Utility OS/VS2 JES2
OS/VS2 JES3
With Systems Network Architecture/Synchronous Data Link Control, the 5280 can act as a secondary station for System/370, 3031, 3032, 3033, 4331, and 4341 processors:
By using
SNA Data Communications Utility
BSC: Binary synchronous communications
SNA/ SDLC: Systems Network Architecture / Synchronous Data Link Control
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